1. Field of the Invention
This invention relates to an improvement of a threaded fastener, of a self-tapping screw, and more particularly to a threaded fastener structure having a plurality of spaced projections on equivalent height thread convolutions of a self-tapping screw threaded fasteners for fastening resilient workpieces.
2. Description of the Prior Art
In recent years, it has been desired to provide an improved fastener to meet the requirements for using resilient plastic material parts or the like in cars in order to assure light weight of the car and to reduce fuel costs in the automotive industry.
A tapping screw for plastic parts or the like has been provided as disclosed in the Japanese patent application publication No. 27504/79. In the conventional fastener shown in FIG. 1A and FIG. 1B, a tapping screw is formed with an eccentric screw thread 20 on a shank 40 and a head 30. The eccentric screw thread 20 is gradually reduced from maximum height to minimum height per one pitch in accordance with a helical line of the screw so that the minimum height becomes even with the outer surface of the blank of the shaft 40, and both of the blank 21 and the maximum height 22 of screw thread 20 are axially and alternately arranged longitudinally on the shank 40.
However, when the screw is fastened in the workpieces, contact area of the screw against the workpieces increasingly spreads, and therefore, large torque is required for turning the screw and after fastening, restoring efficiency of the extended overly wide area of the tapped portion of the workpieces is poor and the fastening force is weak, since the eccentric screw thread 20 of the tapping screw is formed to a slight grade and is projected at all thready sections except at the minimum height at the blank 21.